Recently, various zoom lenses having a high zoom ratio or high optical performance have been proposed. These zoom lenses include at least three movable lens groups (wherein these zoom lenses are called "multiple-movable-group zoom lenses"). Multiple-movable-group zoom lenses have been made possible by recent advances in lens barrel technology.
In recent years zoom lenses with compact size have been proposed to make a combination of zoom lens and camera body as small as possible. Specifically, zoom lenses such as a positive-positive-negative three-group zoom lens and a positive-negative-positive-negative four-group zoom lens have been proposed.
Generally, in multiple-movable-group zoom lenses, flexibility in selecting the zooming locus of each lens group is increased, so the flexibility to correct various aberrations is increased. Moreover, as the number of movable lens groups is increased, the change in the lateral magnification of each lens group is decreased in order to increase the zoom ratio and the performance of the zoom lens. With recent advances in lens-barrel technology, problems such as an increased complexity of the lens-barrel structure accompanying an increase in the number of axially movable lens groups have been overcome to a certain extent.
Until recently, in zoom lenses which are not restricted in back focus, a negative lens group was arranged as the rearmost (i.e., most imagewise) lens group and a positive lens group was arranged as the frontmost (i.e., most objectwise) lens group. The change in total length and back focus of the zoom lens were increased to minimize the zoom lens.
Each lens group in a conventional positive-positive-negative (from the object side to the image side) three-group zoom lens is axially movable objectwise such that, during zooming from the wide-angle end to the telephoto end of the zoom lens, the air space between the first and second lens groups axially expands and the air space between the second and third lens groups axially contracts. Such a lens is disclosed, for example, in Japan Laid-Open Patent Application No. Hei 2-256015.
Each lens group in a conventional positive-negative-positive-negative (from the object side to the image side) four-group zoom lens is axially movable objectwise such that, during zooming from the wide-angle end to the telephoto end, the air space between the first and second lens groups axially expands, the air space between the second and third lens groups axially contracts, and the air space between the third and fourth lens groups axially contracts. Such zoom lenses are disclosed, for example, in Japan Laid-Open Patent Application No. Sho 60-57814 and Japan Laid-Open Patent Application No. Hei 5-150161.
In a lens-shutter type of camera with a zoom lens, the lens system is retracted into the camera body as each variable air gap is minimized during zooming. Optional retraction of the lens requires corresponding reductions in the axial thickness of each lens group of the zoom lens.
In conventional three-group (positive-positive-negative) zoom lenses as disclosed in Japan Laid-Open Patent Application No. Hei 2-256015, the change in the lateral magnification of the third lens group during zooming is large. As a result, the variation of off-axis aberrations arising in the third lens group during zooming is increased for zoom lenses having a high zoom ratio (i.e., greater than 2.5). An additional disadvantage with such zoom lenses is that, because the second lens group typically comprises a negative lens group and a positive lens group, the number of constituent lens elements in such lenses is disadvantageously large.
In conventional four-group (positive-negative-positive-negative) zoom lenses as disclosed in Japan Laid-Open Patent Application No. Sho 60-57814, the combined refracting power of the first and second lens groups at or near the wide-angle end is close to zero. As a result, it is difficult to obtain a sufficient back focus with such lenses at the wide-angle end. Also, since off-axis rays through the fourth lens group tend to be away from the axis, the diameter of the fourth lens group would need to be too great.
A four-group zoom lens is favorable for achieving a high zoom ratio, but the change in the off-axis ray height at the second lens group during zooming is small, though the change in the incident angle of the off-axis ray to the second lens group is large. Consequently, when the zoom ratio is greater than three, the variation of the off-axis aberration arising in the second lens group increases during zooming and it is difficult to obtain high performance.
In the four-group (positive-negative-positive-negative) zoom lens disclosed in Japan Laid-Open Patent Application No. Hei 5-150161, the combined refractive power of the first and second lens groups at the wide-angle end is negative. However, the change in the total length of the zoom lens during zooming is small, so such a zoom lens is not suitable for achieving a high zoom ratio.
Therefore, an object of the present invention is to provide a zoom lens having a high zoom ratio while maintaining good performance, simple construction (including the smallest possible number of lens elements), and compactness.